(1) Field of the Invention
The present invention relates to two-wire AC switches, and more particularly to a two-wire AC switch including a bidirectional switch element made of a group-III nitride semiconductor.
(2) Description of the Related Art
Conventionally, mechanical switches have been widely used as two-wire AC switches to control lighting apparatuses, ventilating fans, and the like in houses. In recent years, there is a need for a bidirectional AC switch that conducts passing current bi-directionally to have high breakdown positive and negative voltages. High-function switches, such as a Triode AC (TRIAC) switch, which are made of semiconductors and have a photochromic function, a remote control function, a human sensing function, and the like, have therefore been commercialized. Japanese Examined Patent Application Publication No. 58-56477 discloses such a high-function switch.
FIG. 12 illustrates a schematic circuit diagram of a conventional two-wire AC switch 151 using a TRIAC as a primary switch. As illustrated in FIG. 12, the conventional two-wire AC switch 151 includes a TRIAC 150 and a drive circuit 149 that drives the TRIAC 150. The TRIAC 150 is connected in series with a commercial AC power supply 101 and a load 102, thereby forming a closed-loop circuit. The drive circuit 149 switches the TRIAC 1500N/OFF to cause the commercial AC power supply 101 to supply or block power to the load 102.
However, when such a bidirectional switch element is a semiconductor device made of silicon (Si), the material limit of silicon makes it difficult to further reduce on-resistance of the bidirectional switch element. More specifically, as seen in a graph of TRIAC on-state characteristics in FIG. 13, the TRIAC 150 used as a bidirectional switch operates in the on-state in the same manner as the bipolar device that has the same on-stage characteristics as those of a silicon diode. Therefore, the TRIAC 150 has high on-resistance in a low current region. As a result, the two-wire AC switch using the TRIAC has a large conduction loss due to the on-resistance. For the above reason, there is a difficulty in using the conventional two-wire AC switch for a load of 200 W or more. Therefore, the use of the conventional two-wire AC switch imposes restrictions on power consumption of available loads.